Seismology makes an important contribution toward monitoring compliance with
the Comprehensive Test Ban Treaty (CTBT). One task at the testbed
of the Center for
Monitorning Research (CMR, Washington DC, USA)
and the International Data Center (IDC) of the
Comprehensive Test Ban Treaty Organization (CTBTO, Vienna, Austria) is the
detection, location and characterization of seismic events in order to
distinguish between possible nuclear tests and earthquakes or other
natural sources of seismic signals. While this is not particularly
difficult for large events, whether natural or man-made,
small events present
a greater challenge. Although their epicenters and magnitudes can be
determined fairly precisely,
seismic moment tensor analysis can help in two ways.
It not only gives information
about the size and mechanism of a
source in terms of its seismic moment and the moment tensor components.
It provides, in addition, an estimate of the source's depth,
which cannot always be reliably determined using normal location
techniques.
Thus, a large non double-couple component ()
may be an indication for a nuclear explosion, as compared to the typically more
than 70-80% double couple for an earthquake (Dreger and Woods, 2002).
The source depth determined from moment tensor analysis may also help to
weed out tectonic events from among the more than 100000 events of
magnitude 4 and greater that occur annually. Only
events at shallow depths need be scrutinized by the
monitoring process of the Comprehensive Test Ban Treaty (CTBT).

This project's goal is to implement the process for
automatic determination of seismic moment tensors routinely used in real-time
at the University
of California at Berkeley (UCB, Romanowicz et al., 1993;
Dreger and Romanowicz, 1994; Pasyanos et al., 1996) on
the testbed at CMR. Although
the moment tensor process will not be running in real-time on the testbed,
in its final implementation it will run automatically, triggered
from the Reviewed Event Bulletin (REB). Thus, it will
be an additional, potentially powerful event screening procedure
(Pechmann et al., 1995; Dreger and Woods, 2002),
providing estimates of: (1)
the moment magnitude, , a very accurate measure
of event size; (2) the source depth,
which will help distinguish natural events with typical depths
greater than than 1 km
from nuclear explosions and (3) radiation
characteristics, such as deviations from the typically double-couple
radiation of earthquakes.

The automated procedure developed at UCB and implemented at CMR
uses two methods for determining the moment tensor.
One is a time domain, waveform fitting procedure that utilizes the
complete, long-period recordings (CW, Dreger and Romanowicz, 1994;
Pasyanos et al., 1996; Fukuyama et al., 1998; Fukuyama
and Dreger, 2000). The other tensor method fits the
surface waves in the frequency domain (SW). It is adapted from the two-step
method of Romanowicz (1982).

During the past year, we have completed the installation of the moment
tensor codes on the testbed at CMR. The software package now automatically
extracts event information and waveform data from the database there,
performs basic quality control and preprocesses the waveforms
before running the two inversions to produce independent
solutions. As testing has proceeded, we have improved the Greens
functions produced for the CW method by applying a flattening algorithm to
the radially symmetric velocity structures (Müller, 1973,
Müller, 1977). We have also adapted the set of
periods used for the SW inversion from those used for
the regional application in California to for application world-wide
on intermediate-sized events.

We have applied the procedure to events shallower
than 200 km with in a test dataset,
the 90 day interval between from July 19, to October 17, 1999. For the event in
Greece on September 7, we have investigated the use of data from auxiliary
stations of the IMS network in addition to the primary stations. Figure
29.1 shows results for the mainshock ( 6.0)
as well as two aftershocks (evt2 5.6 and
evt3 4.8). Clearly, the method is effective in this region, even
for the small aftershock.

For events in the test dataset
we have run inversions using two
different velocity models. The maps in Figure 29.2 show
the IMS stations
used for the inversions,
as well as the moment tensor solutions determined by the complete
waveform inversion and the surface wave method, respectively. In both
Figure 29.2 A and B,
the solutions derived using two different velocity models are
compared with the moment tensors
given in the Harvard
CMT and USGS catalogs.

While the match between catalog source mechanisms and
those calculated using the two automated moment tensor methods is
good for some events, for others the process is
not so successful. One typical problem is that for this
interval, data is not always available from many of the primary
stations of the seismic network of the International
Monitoring Systems (IMS), the data source for the automatic process.
For the CW method, for example, the moment
tensors derived for
events east and northeast of Australia differ from those given by
both the Harvard and USGS catalogs. However,
for each of these events, data were
only available from one primary station less than 5000 km
from the epicenter, STKA. The solutions calculated by
the CW method are consistent with the waveforms from this
station. The dearth of data is apparent for the SW method in
in Figure 29.2 B which shows
solutions for only 13 of the
19 events shown in Figure 29.2 A.

Currently, we are directing our efforts toward three fronts. First,
we will attempt to improve the automated procedure by incorporating
data from additional stations. Since 1999, the primary stations of
the IMS network have been improved, both in their equipment and in
their reliability. In addition, many of the auxilliary stations of
the IMS network satisfy the need for the broadband, high dynamic
range data which is necessary for the methods to work well. We will
factor in data from these stations to improve the solutions.
Secondly, we are working to develop and apply quantitative
comparisons of the moment tensor solutions from various sources,
CW or SW methods, as well as Harvard CMT and USGS. Finally, we
are developing a regionalized calibration for the Far East. As part
of an advanced concept demonstration, the CMR
has collected event information and seismograms,
as well as information
about the Earth's structure in the region around Lop Nor. We will
use this data to generate Greens functions and path information, so
that we can calculate moment tensors for events, man-made or natural,
occurring in this area.

Figure 29.2:
(A) Map showing stations (triangles) and
inversion results for the complete waveform method.
Taken in order from the event hypocenter (dot),
focal mechanisms are from the Harvard
CMT catalog, USGS catalog, CW method using iasp91 velocity model and
CW method using PREM. (B) Map showing stations (triangles) and
inversion results for the surface wave method.
Taken in order from the event
hypocenter (dot), focal mechanisms are from the Harvard
CMT catalog, USGS catalog, SW method using m1066b velocity model and
SW method using PREM.